Using multiprocessing to speed up multiple sound bakings
I'm sure someone will school me if I'm wrong (please do!):

The Python multiprocessing module allows one to run concurrent python code, this makes sense for processes that don't rely on each other to reach a conclusion. What you want to do is get Blender to concurrently perform the sound bake function (C code). As far as I'm aware calls to that function are blocking Blender from doing anything else until the function ends and control is returned to python. 

If that's right, then A solution could be the one I offered in the comment s:


use Popen or subprocess and instantiate multiple Blender threads with scripts, and  append the F-curves from each resulting .blend into a final .blend.